This invention relates to a method of controlling a communication system, and particularly to enable interrogation of other channels in a radio system so that a handover request may be generated. In particular, the invention is concerned with handover from an established channel operating in a frequency
The need for handover control in cellular systems is well known, and many techniques for changing the base station with which a mobile unit is communicating have been proposed. The present invention is particularly concerned with the handover of communication signals between different systems, or within a single system which supports more than one frequency channel or different types of communication in different areas.
For a mobile station to determine when handover is appropriate, some monitoring must be made of alternative available channels so that a correct handover decision can be made. A particular problem arises if an existing communication channel is operating in a CDMA frequency division duplex (FDD) mode, because user data is assigned to virtually the full duration of each time frame of the communication channel, so that there is no available time for the mobile station to monitor alternative communication channels. One possible solution to this problem is to provide each mobile station with two receivers, one for receiving the user data transmitted by the base station, and the other for monitoring the condition of other possible channels. It is, of course, desirable to avoid the need for each mobile station to have two receivers.
An alternative approach is to provide a slotted transmission mode in which for some or all time frames, the base station transmits all of the data for the frame, but in part of the frame duration. This would normally require that the information rate is increased during transmission, for example by reducing the spreading factor in a spread spectrum system. No information is sent to the mobile station during the remainder of the frame, which comprises a null period. There could be information sent in the null period, but of a nature which can be ignored by the mobile station. The null periods enable the mobile station to use a single receiver to perform signal measurements during the null periods in order to analyse alternative channels or frequency bands. This analysis then enables evaluation of whether handover is appropriate.
WO 94/29981 discloses a transmission system for handover in DSCDMA systems, in which idle periods are provided in the otherwise continuous transmission. In a time frame having an idle period, the transmission of the channel data in the time frame is carried out with a higher transmission power, and with a lower spreading ratio. The idle periods enable other channels to be monitored by the mobile station to enable seamless handover.
A problem with the system of WO 94/29981 is the need to dynamically alter the spreading ratio of the CDMA coding. A further problem may arise concerning the timing of the idle periods.
For example, in order to evaluate whether an alternative channel is appropriate for handover, the mobile station needs to obtain information concerning other channels, which information may be transmitted only at specific times. For example control data defining a channel may be provided at an allocated timing within each time frame, for example in the header of each time frame. A system operating using time division duplex (TDD) will only produce base station transmissions during a fraction of the time slots in a time frame. Therefore, for the mobile station to evaluate whether an alternative possible channel is appropriate, it needs to interrogate alternative channels at specific but unknown times.
A particular problem may arise if the time frame duration of the alternative channel is the same as the time frame duration of the channel being operated, because the null periods during which the mobile station interrogates alternative channels may not correspond with the control data which is required to make a handover decision.
According to the preset invention there is provided a method of controlling a communication system in which a communication signal is established between first and second stations and using a channel configuration comprising sequential time segments with the channel defining a substantially continuous transmission, comprising:
dividing each time segment of the channel from the first to the second station into a predetermined number of sub-segments;
for a first set of time segments, repeating the communication signal data for a respective time segment in all sub-segments; and
for a second set of time segments, providing in one or more of the sub-segments null periods during which the communication signal from the first station to the second station is interrupted, and transmitting the communication signal data during the remaining sub-segments of the second set.
The use of some sub-segments as null periods enables the transmitter of the first station and/or the receiver of the second station to be used for other purposes, for example for establishing handover. The first station may be the mobile or the base station, and correspondingly the second station may be the base or the mobile station. The time segments preferably comprise time frames. The use of a segmented time frame structure, even for time frames in which no null periods are provided, enables a constant spreading factor to be applied in the case of a code division transmission system. However, for time frames with repeated data (at high bit rate), the data can be combined by the receiving terminal with little or no increase in error occurrence compared to a conventional lower bit rate transmission. Of course, the bit rate of the signal for transmission does not necessarily need to be constant, for example considering variable rate speech codecs.
The transmission during the at least some of the remaining sub-segments for each time segment in the second set may be performed with an increased transmission power, so that the received signal has comparable error characteristics.
Null periods are preferably also provided in a channel from the second to the first station, some of which may be synchronised with null periods in the channel from the first to the second station. This will minimise interference during handover channel monitoring.
The second station may be operated to survey alternative communication channels during the null periods, and to generate a handover request in response to the survey, if a more appropriate channel is identified during the survey. The null periods may be provided in different sub-segments for different time segments in the second set. This overcomes the problem that the null period may repeatedly miss the required data portion of alternative channels being monitored for handover.
The invention also provides a telecommunication system employing a method as described above.
The invention further provides a telecommunication station for transmitting a signal over an allocated channel which is divided into time segments, comprising transmitting means, timing means for dividing the time segments into sub-segments, repeating means for allocating communication signal data to a plurality of sub-segments, and interrupting means for arranging one or more sub-segments as null periods during which transmission is interrupted, wherein for a first set of time segments the repeating means allocates the communication signal data to all sub-segments for transmission at a first power, and wherein for a second set of time segments the interrupting means arranges one or more sub segments of each time segment as a null period and the repeating means allocates the communication signal data to the remaining sub-segments of the second set for transmission at a second power which is greater than the first power.
This transmission station is thus capable of operating according to the method of the invention. The signal for transmission is preferably spread by a code sequence, and wherein the same spreading factor is applied to time segments having sub-segments arranged as null periods as to time segments in which no sub-segments are arranged as null periods.
The invention additionally provides a telecommunication station for receiving a signal from a transmitting station over an allocated channel which is divided into time segments with the time segments divided into a predetermined number of sub-segments, comprising receiving means, means for interrupting operation of the receiving means during sub-segments arranged as null periods, means for combining data in sub-segments arranged as repeated data of individual time segments, and means for operating the receiving means to survey other communication channels during the null periods.
This telecommunication station is thus capable of operating according to the method of the invention. The telecommunication station preferably further comprises de-spreading means which utilises the same spreading factor for time segments having sub-segments arranged as null periods as for time segments in which no sub-segments are arranged as null periods.